Extrusion method for high strength heat treatable aluminum alloys

ABSTRACT

A method for extruding high strength, heat treatable aluminum alloys is disclosed which comprises providing a homogenized cast alloy billet, conducting a first hot extrusion of said billet to a reduction in area of from 20 - 75%, conducting a second hot extrusion at the solutionizing temperature of the alloy, and quenching said billet after said second extrusion. The method of this invention is particularly useful for the processing of Alloys 2014, 2024 and 7075, and eliminates the need of conducting a separate solution heat treatment after the extrusion operation is completed.

Sperry et al.

EXTRUSION METHOD FOR HIGH STRENGTH HEAT TREATABLE ALUMINUM ALLOYSInventors: Philip R. Sperry, North Haven;

William C. Setzer, Hamden; Joseph Winter, New Haven; Michael J. Pryor,Woodbridge, all of Conn.

Swiss Aluminium Ltd., Chippis, Switzerland Filed: May 23, 1974 Appl.No.1 472,656

Assignee:

US. Cl 72/364, 72/700, 148/1 1.5 A Int. Cl. B21c 29/00 Field of Search72/364, 700; 148/1 1.5 A

References Cited UNITED STATES PATENTS 7/1941 Fritzlen l48/l1.5 4/1962Criner l48/ll.5

[451 Apr. 1, 1975 3,113,052 12/1963 Schneck 148/1 1.5 3,307,978 3/1967Foerster... 148/] 1.5 2/1974 Kroger l48/ll.5

Primary ExaminerLowell A. Larson Attorney, Agent, or Firm-David A.Jackson; Robert H. Bachman [57] ABSTRACT A method for extruding highstrength, heat treatable aluminum alloys is disclosed which comprisesproviding a homogenized cast alloy billet, conducting a first hotextrusion of said billet to a reduction in area of from 20 75%,conducting a second hot extrusion at the solutionizing temperature ofthe alloy, and quenching said billet after said second extrusion. Themethod of this invention is particularly useful for the processing ofAlloys 2014, 2024 and 7075, and eliminates the need of conducting aseparate solution heat treatment after the extrusion operation iscompleted.

9 Claims, No Drawings EXTRUSION METHOD FOR HIGH STRENGTH HEAT TREATABLEALUMINUM ALLOYS BACKGROUND OF THE INVENTION This invention relates to aprocess for extruding and heat treating high strength aluminum alloyswithout the need of separate solution heat treatment after extrusion.

It has long been the practice to heat extrusions of heat-treatablealuminum alloys by a process known as solution heat treatment, toachieve the desired temper. Such treatment involves heating theextrusion to a temperature at which solution and diffusion of theheattreatable alloying constituents take place and produces, as nearlyas practicable, a homogeneous solid solution. The extrusion issubsequently quenched (i.e., rapidly cooled) in order to prevent thehardening constituents from precipitating substantially from solidsolution during the cooling period. Slow cooling, on the other hand,would permit these constituents to precipitate to a greater extent, sothat the alloy would be in a partially annealed condition unsuitable forsubsequent precipitation heat treatment. Solution heat treatment,including the quench, is considered a necessary preliminary tosubsequent precipitation heat treatment to increase the mechanicalproperties of the alloy.

Various methods have been proposed in the art to eliminate the need forpost-extrusion solutionizing of aluminum alloys, however, no method isknown which has been particularly successful when employed with the highstrength aluminum alloys, such as Alloys 2014, 2024 and 7075. Thesealloys possess certain characteristics which prevent the successfulreduction of the solutionizing treatment, among them, a relatively smallcritical temperature range between solvus temperature and solidustemperature, quench sensitivity causing loss of strength and corrosionproblems, and susceptibility to hot cracking when hot worked atsolutionizing temperatures due to the inherent heterogeneous nature ofthe near-cast structure. Of the above characteristics, the first inheresin the specific composition of the alloy and cannot be changed, and thesecond is a function of the physical means available for effecting aquench, and may be overcome by the choice of a suitable system fromamong those that exist in the art. The third characteristic, however,has proved most difficult to overcome, and, it is the solution to theproblem posed by this third characteristic with which the method of thisinvention is primarily concerned.

SUMMARY OF THE INVENTION The method of this invention relates to aprocess for extruding and heat treating high strength aluminum alloys,without the need of a separate solution heat treatment and quenchingafter extrusion, by the use of a prior hot extrusion preceding the finalextrusion operation.

The method of this invention facilitates the successful extrusion ofhigh strength heat treatable aluminum alloys at commercial temperswithout the employment of an additional solutionizing treatment afterfinal extrusion. The product obtained by the method of this inventioncan be hot worked at their solutionizing temperature ranges withoutbreaking up, and can develop characteristic mechanical strengthproperties when quenched and aged.

Accordingly, it is a principal object of this invention to provide amethod for the preparatioin of high strength aluminum alloys byextrusion which eliminates the need of separate solution heat treatmentafter extrusion.

It is a further object of this invention to provide a method asaforesaid which is both economical and time saving.

It is yet a further object of this invention to provide a method asaforesaid whereby extrusion can be conducted in solutionizingtemperature ranges without causing break-up of the alloy duringextrusion.

lt'is yet a further object of this invention to provide a method asaforesaid by conducting a prior hot extrusion preceding the finalextrusion of the alloy.

Other objects and advantages of the present invention will becomeapparent from the detailed description which follows.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with thepresent invention, it has been found that the foregoing objects andadvantages can be readily achieved, and that high strength aluminumalloys may be successfully extruded to commercially acceptable tempers.

The method of this invention comprises an extrusion operation for highstrength heat-treatable aluminum alloys which eliminates the necessityof a postextrusion solution heat treatment by providing a hot extrusionof the cast billet to a reduction in area from 2075% preceding the finalextrusion operation.

The method of this invention may be employed with a broad range ofaluminum alloys, and is particularly useful with high strength heattreatable alloys, such as those designated by the Aluminum Associationas Alloys 2014, 2024 and 7075. The compositions of these alloys arepresented in Table I, below.

All of the values for the constituents set forth above are stated on apercentage part by weight basis, and represent maximum amounts unlessgiven in a range.

It has been found in accordance with this invention that these highstrength heat treatable alloys may be extruded at solutionizingtemperatures without causing break-up of the extrusion surface, andwithout the need of either reduced extrusion speeds or a previoussolution heat treatment.

Before commencing the practice of the method of this invention, a castand homogenized billet of suitable diameter is prepared. The proceduresof casting and homogenization are not critical and may be followed inaccordance with conventional practice in the art.

The billet is then reheated to a temperature of from about 700850 F.This heating must be uniform so that all parts of the billet are withinthe stated temperature range upon completion.

Upon completion of the reheating step, the billet is then extruded to asmaller diameter to obtain a reduction in area of about -75%. Such areduction range would correspond to extrusion ratios of from about1.25:1 to 4:1. This prior'hot working at a lower temperature than theconventional solutionizing extrusion step serves to break up andelongate the cast grain so that the alloy can better withstand thedeformation at the subsequently applied high temperatures. Extrusionwithin the range of reductions set forth for the above would, at most,serve to reduce the diameter of the billet by about 50% and theresulting billet would remain large enough to undergo furtherextrustion.

Upon being partially extruded in the above manner, the billet is thencut to a usable length and reheated to a temperature in a rangegenerally residing near the lower end of the solutionizing temperaturerange. The solutionizing temperature range varies with the particularalloy in question and thus, for example, the lower end of thetemperature range would be 910 F for Alloy 2024, 925 F for Alloy 2014and 860 F for Alloy 7075. The reheating range should be controlled toextend from an upper limit approximated by the solutionizing temperatureranges, above, to a temperature level roughly 30 F lower. This lowerrange is feasible because adiabatic heating during extrusion can causesome temperature rise to solutionizing temperatures and, in the eventthat solutionizing is not complete, the extrusion effect serves toprovide extra strength to the alloy which compensates for the resultingloss of solutionizing.

The control of the reheating step is considered critical, since, asnoted earlier, the alloys in question possess a relatively smalltemperature range between solvus and solidus. Further, reheating must beconducted for a time sufficient to dissolve substantial amounts ofsoluble elements present within the alloy. The ability to control bothreheating temperature and residence time are dependent upon the methodof heating employed. Two methods of billet heating are presently ingeneral use. Induction heating affords a rapid rise in temperature, andthe desired temperature control, but is usually not maintained at thattemperature for a time sufficient to constitute an effectivesolutionizing treatment. A conveyor-type furnace, however, which isheated electrically or by gas flame, provides a longer heating time;however, the billet surface is usually directly exposed to the heatsource elements, increasing the danger of overheating the surface withthe result that partial melting may occur. This latter danger isespecially great with high strength aluminum alloys having little spreadbetween solvus and solidus temperatures. In order to provide adequatecontrol and duration of reheating, a modified induction heating methodmay be employed which makes use of a controlled time-hold cycle and, inthe alternative, combines induction heatup with a hold in a circulatingair furnace with a separate plenum chamber to isolate the heat sourcefrom the 'billet metal.

Upon completion of reheating, the billet is then extruded in theconventional manner at a solutionizing temperature with the limitation,however, that the extrusion rate is controlled to prevent a rise intemperature during extrusion of more than about 20 F above the minimumsolutionizing temperatures, described earlier. To this end, thetemperature of the extrusion container or canister should only bepermitted to rise higher than about 25 below billet temperature. Thecriticality of temperature control during extrusion is likewisenecessitated by the temperature sensitivity of the high strengthaluminum alloys involved. Temperature control in this instance is afunction of the interrelationship between heat generated by the workingprocess itself and heat conducted away by the extrusion tools andsurroundings. Thus, extrusion speed must be regulated to preventunacceptable temperature fluctuations and, as noted above, the containertemperature should be well controlled. The actual temperature limits andthe speed of extrusion, however, will depend on the individual extrudedshape and the alloy involved and, therefore, cannot be rigidly definedherein.

Two other factors which contribute to successful extrusion are theextrusion ratio and extrusion speed. The extrusion ratio was determinedwith reference to a single hole die, and should not exceed about 30:1.In conjunction with this, the length-diameter ratio of the billetsshould not exceed 2521. The factor of extrusion speed is interrelatedwith the former two, in that extrusion speed is in part determined bymagnitude of the extrusion ratio. Control of extrusion speed isimportant since excessive speed tends to cause break up of the extrusionas it exits from the die. In experiments dealing with the alloysrelating to this invention, extrusion speeds of up to about 4 ft./min.were found to be acceptable, however, the process is not restrictedthereto, provided the desired temperature limits are obtained andcracking does not occur.

The finally extruded shape must then be quenched, in accordance withconventional practice in the art. The specific nature of the quenchingoperations, however, is critical, for, as noted before, the alloys whichare to be extruded by the method of this invention process quenchsensitivity and are known to exhibit loss of strength and corrosionresistance. Thus, the quench must be started before the alloy has cooledenough to lose some of its solid solution, and must be rapid enough toassure than adequate mechanical strength and corrosion resistance areretained. In a preferred embodiment, quenching is conducted continuouslywith the extrusion moving through a water wall. However, because of thestop and start nature of the extrusion operation, it may be desirable tomaintain the entire extruded length at the solutionizing temperaturerange while it is being extruded and cut off or removed from the die.This can be accomplished by the use of an insulating tunnel or somesimilar apparatus. After cutting, the entire extrusion may be quenchedby immersion in a horizontal through or by passing it through watersprays and the like. As an alternative, a moving saw may be placed asclose as possible to the die face in order to quickly sever theextrusion and allow it to pass through a quench system without stopping.The determination of specific quenching time will vary with the alloyinvolved and the attendant extrusion conditions and need not be furtherdeveloped at this time.

Upon emerging from the quench, the extruded alloy may be subjected toconventional processing such as stretch or roll straightening, naturalor artificial aging depending upon the final temper which is desired.Thus, for example, if it is desired to prepare Alloy 7075 in T-6 temper,the quench shape may be subjected to aging treatment which may beconducted at a temperature of about 250 F for about 24 hours. Such anaging treatment, however, is applicable to specific alloys for whichparticular temper is sought and may vary considerably in relation to theparticular alloy treatment desired.

As an illustration of the method of this invention, one may, forexample, prepare or otherwise obtain cast and homogenized billets of theaforementioned aluminum alloys, and expose them to a heat treatment in agas fired furnace with controlled heat zones to raise their temperatureto about 800 F, for a period of about 25-40 minutes. The heated billetsmay then be extruded on an extrusion press which employs a circular dieof 8 inch diameter and an extrusion ratio of approximately 2.25:].

The extruded cylindrical shape emerging from such extrusion may then becut to length to serve as billet for final extrusion. It is firstreheated to a temperature level which approximates the solutionizingtemperatures of the respective alloys, and subsequently extruded in a3,000 ton extrusion press which employs a die having an extrusion ratioof 25:], at a surface speed of from about 23 ft./minute for alloys suchas Alloy 2024, and 1-2 ft./minute for alloys such as Alloy 7075. Uponexisting from the end of the extrusion press, the extrusion enters awater wall quenching through to effect the rapid cooling which mustfollow solutionizing.

The alloy products prepared by the above extrusion method are not proneto cracking and break-up during processing, and may be further processedto commercial tempers. Thus, for example, extrusions of Alloy 7075 maybe treated at 250 F for 24 hours to achieve T-6 temper for that alloy.Extrusions of Alloy 2024, if left in the naturally aged condition, areequivalent to T-4 temper.

The above processing scheme is outlined for purposes of illustrationonly and should not be construed as limitative of the invention.

This invention may be embodied in other forms or carried out in otherways without departing from the spirit or essential characteristicsthereof. The present embodiment is therefore to be considered as in allrespects illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims, and all changes which comewithin the meaning and range of equivalency are intended to be embracedtherein.

What is claimed is:

1. A method for extruding high strength, heat treatable aluminum alloyswhich comprises providing a homogenized cast alloy billet, conducting afirst hot extrusion of said billet to a reduction in area of from20-75%, conducting a second hot extrusion at the solutionizingtemperature of the alloy, and quenching the resultant extruded shapeafter said second extrusion.

2. The method of claim 1 wherein said first extrusion provides areduction in area of about 50%.

3. The method of claim 1 wherein the billet is reheated to within therange of about 700850 F prior to conducting said first extrusion.

4. The method of claim 1 wherein, after said first extrusion but beforesaid second extrusion, said billet is reheated to a temperature residingwithin the lower end of the solutionizing temperature range of thealloy.

5. The method of claim 1 wherein said second extrusion is regulated toprevent a rise in the temperature of the billet during extrusion abovethe maximum permissible solutionizing temperature of the alloys.

6. The method of claim 5 wherein said regulation is achieved by controlof extrusion speed and container temperature.

7. The method of claim 6 wherein container temperature is maintained atabout 25 F below the temperature of the billet upon entering saidcontainer.

8. The method of claim 1 including the step of againg the shape aftersaid quenching step is completed.

9. The method of claim 1 including the step of stretch and rollstraightening the shape after said quenching step is cpmpleted.

1. A METHOD FOR EXTRUDING HIGH STRENGTH, HEAT TREATABLE ALUMINUM ALLOYSWHICH COMPRISES PROVIDING A HOMOGENIZED CAST ALLOY BILLET, CONDUCTING AFIRST HOT EXTRUSION OF SAID BILLE TO A REDUCTION IN AREA OF FROM 20-75%,CONDUCTING A SECOND HOT EXTRUSION AT THE SOLUTIONIZING TEMPERATURE OFTHE ALLOY, AND QUENCHING THE RESULTANT EXTRUDED SHAPE AFTER SAID SECONDEXTRUSION.
 2. The method of claim 1 wherein said first extrusionprovides a reduction in area of about 50%.
 3. The method of claim 1wherein the billet is reheated to within the range of about 700*-850* Fprior to conducting said first extrusion.
 4. The method of claim 1wherein, after said first extrusion but before said second extrusion,said billet is reheated to a temperature residing within the lower endof the solutionizing temperature range of the alloy.
 5. The method ofclaim 1 wherein said second extrusion is regulated to prevent a rise inthe temperature of the billet during extrusion above the maximumpermissible solutionizing temperature of the alloys.
 6. The method ofclaim 5 wherein said regulation is achieved by control of extrusionspeed and container temperature.
 7. The method of claim 6 whereincontainer temperature is maintained at about 25* F below the temperatureof the billet upon entering said container.
 8. The method of claim 1including the step of againg the shape after said quenching step iscompleted.
 9. The method of claim 1 including the step of stretch androll straightening the shape after said quenching step is cpmpleted.